[TowerTalk] [RFI] RFI With Smoke Detectors

[Jim Brown]

On Sun,8/23/2015 11:49 AM, Roger (K8RI) wrote:

> The reason for chokes was "it was handy".  Being lazy, I decided to 
> try the second choke where the coax reached the tower as there was 
> already a connector there. The only extra work that entailed was 
> removing the weatherproofing from the connectors.  It worked and that 
> let me get away without adding more weight hanging off the center of 
> the dipole. 

That's a good move. The second choke reduces the common mode current, 
reducing the stress on both chokes.

> With the 5 core, 6 turn choke 
> http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm   plus nearly 
> 100 feet of LMR-400 already hanging off the center. That dipole 
> already has well over 100# of tension to minimize the sag

I've got about 90# tension on mine, in the form of a water jug filled 
with dry sand.  Looking at your photo -- this choke is optimized for 
30-15M. The wide spacing of the turns minimizes capacitance, so the 
resonance is too high for 75M. Below 20M, it's best to closely space the 
fewer number of turns that can be achieved with big coax.

> I am building a single, larger choke, but using the 1/4" coax with a 
> Teflon dielectric and a double shield of silver plated braid. The 
> cores are spaced 1/8th inch and epoxied into the Lexan spacers.  I've 
> not settled on the # of cores or turns of coax. The spacers are strips 
> of 1/4" Lexan 3/4" wide by a length to be determined.  The slots are 
> milled 1/8th inch deep with 1/8th in spacing. I did one trial using 
> Plexiglass spacers with 1/4" deep slots, spaced 1/4", but they were 
> too fragile.  The Plexiglass strips were cut from scrap and were 
> free.  The Lexan was cut from a relatively small piece.  Still they 
> were less than $2.00

Going to the Teflon coax is a good move in your system, because it 
allows the greater number of turns needed for an effective choke. I have 
not measured chokes wound with this type of coax -- the dielectric 
affects the capacitance between turns, which can shift the resonance a 
bit. I suggest that you follow my cookbook for RG8X-size coax.

>
> I'll have to shoot some photos of the spacers and the construction.

Nice looking, Roger. You say 75M fan -- I'm assuming multiple wires to 
increase the SWR bandwidth on 75M only.  My long fans are 2-band, with a 
single wire for 80 and a second for 40.

73, Jim K9YC